The use of sensors to identify and track objects is well known in the prior art. Some sensors, such as radar systems, send out signals that reflect from objects and are received by the system. Other sensors, such as electro-optical sensors using telescopes and focal plane arrays, receive electromagnetic radiation signals from the objects themselves. Refining these sensors to be ever more accurate is ongoing development in this field.
One major area of development, especially in optical sensors using telescopes and focal plane arrays that detect infrared radiation, is the suppression of fixed pattern noise. The most common way disclosed in the prior art is to use calibration or non-uniformity correction. During calibration, the focal plane array receives radiation from a first uniform gray screen at one intensity and then from a second uniform gray screen of an another intensity. Based on the two sets of received data, a gain and a corresponding offset is determined for each detector in the focal plane array. Then, if the focal plane array does not change significantly, an inverse correction for the gain and the offset can be used to provide a useful degree of suppression of the fixed pattern noise. However, as time passes, the focal plane array does change and the calibration quality deteriorates. Further, the correction is only useful for operating conditions (intensity, spectral distribution, and focal plane array temperature) that remain reasonably close to those for which the calibration was performed. This method of fixed pattern suppression, particularly in a long wavelength infrared sensor, normally leaves a large residual fixed pattern which limits the sensor performance, often increasing the sensor noise level by an order of magnitude or more above the level that could otherwise be achieved.
Tracking objects using an optical sensor with telescope and focal plane array on a moving platform presents additional problems, including compensating for the movement of the moving platform and/or the target, also know as stabilization. The prior art discloses the use of position compensating devices, such as scan mirrors or gimbals equipped with servo drives and position pickoffs, to compensate for the movement of the platform. The position compensating devices receive input from reference gyros, either alone or contained within an inertial measurement unit (xe2x80x9cIMUxe2x80x9d), which comprises a triad of gyroscopes and a triad of accelerometers. From the gyro data, the position compensating devices mechanically stabilize the optical sensors to compensate for the movement of the platform. However, the position compensating devices are complicated (both mechanically and electrically), expensive, and their weight and volume is onerous.
The prior art also discloses using multiple focal plane arrays to detect electromagnetic radiation of different wavelengths. For example, two separate focal plane arrays have been used to detect a long wave infrared band and a short wave infrared band. In a typical arrangement, a dichroic beamsplinter is used to separate the two different wavelengths. For example, the dichroic reflects the short wave band to the short wave focal plane array while it transmits the long wave band energy to be received by the long wave band focal plane array. The use of two focal plane arraysxe2x80x94or more if multiple wavelength bands are desiredxe2x80x94increases the complication, size, and expense of the system. Therefore, for moving platform applications, an optical sensor is needed that does not require mechanical stabilization, that has fixed pattern suppression, that does not require focal plane calibration and that does not limit the performance of the sensor, and has only one focal plane array but detects multiple wavelength bands.
In an aspect of the invention, a motion compensated integration system for scanning a field comprises a moveable platform, an optical sensor, and optical sensor line-of-sight measuring device, and a processor. The optical sensor is mounted to the moveable platform and comprises a telescope in functional relationship with a focal plane array. The focal plane array comprises one or more sets of detectors capable of receiving one or more wavebands and producing focal plane array data of the wavebands, respectively. The optical sensor line-of-sight measuring device is capable of producing optical sensor line-of-sight movement data. The processor is capable of receiving the focal plane array data and the optical sensor line-of-sight movement data and producing output images representing the wavebands, respectively.
In a further aspect of the invention, the focal plane array comprises a plurality of sets of detectors and the sets of detectors are intermingled to form a pattern. In still further aspects of the invention, the pattern may be stripes, checkers, zigzags, or random. In an additional aspect of the invention there are two sets of detectors and the pattern is alternating stripes.
In a further aspect of the invention, the processor is enabled to perform the steps of:
a. receiving repeated scans of the field from the focal plane array resulting in scanning data for each detector;
b. generating a subframe array from the scanning data for each waveband for each scan performed, resulting in a total number of subframe arrays;
c. generating offset movement data of the focal plane array corresponding to an approximated line-of-sight for each subframe array;
d. integrating the subframe arrays into a stabilized waveband array for each waveband using the offset movement data, wherein the stabilized waveband array is comprised of elements;
e. estimating a fixed pattern for each detector by integrating the scanning data for each detector and dividing the integrated scanning data by the total number of subframe arrays;
f. suppressing the fixed pattern for each detector from the stabilized waveband arrays by deducting from each stabilized waveband array element the fixed pattern for each detector for each incidence that the detector contributed to each stabilized waveband array element, thereby generating fixed pattern suppressed stabilized waveband arrays for each waveband; and
g. outputting the fixed pattern suppressed stabilized waveband arrays as the output images.
In a still further aspect of the invention, the processor comprises machine readable instructions for directing the process to perform the steps a through g listed directly above. In another aspect of the invention, the processor comprises hardwired devices for performing at least a portion of the steps a through g listed directly above.
In a further aspect of the invention, the stabilized waveband arrays are inertially stabilized.
In a further aspect of the invention, the fixed pattern suppressed stabilized waveband arrays are comprised of elements that were contributed to by a contribution number of subframe arrays, respectively. Further, the processor is enabled to perform the further step of normalizing the fixed pattern suppressed stabilized waveband arrays by dividing each fixed pattern suppressed stabilized waveband array element by a respective contribution number.
In a further aspect of the invention, the focal plan array and the processor are designed to scan at 4000 Hz and produce output images at 10 Hz.
In aspects of the invention, the optical sensor is mounted to the moveable platform in any suitable fashion. In an aspect of the invention, the optical sensor is fixedly mounted to the moveable platform. In another aspect of the invention, the optical sensor is movedly mounted to the platform such that a line-of-sight of the optical sensor may be changed relative to the moveable platform.
In aspects of the invention, the moveable platform of the motion compensated integration system is a component of a missile, the telescope, a satellite, a space vehicle, an air vehicle, an aircraft, a ground vehicle, or a watercraft.
In an aspect of the invention, an attitude control system is functionally connected to the moveable platform for maintaining an angular position of the moveable platform with predetermined limits of roll, pitch, and yaw. In a still further aspect of the invention, the processor comprises means for directing the attitude control system to maintain movement of the moveable platform within a predetermined angular velocity range.
In an aspect of the invention, the optical sensor line-of-sight measuring device comprises an inertial reference unit mounted to the moveable platform. The inertial reference unit is capable of producing the optical sensor line-of-sight movement data and the processor is capable of receiving the optical sensor line-of-sight movement data. In an aspect of the invention, the inertial reference unit comprises one or more gyroscopes.
In an aspect of the invention, the system does not comprise scan mirrors, gimbals, or position pickoffs.
In an aspect of the invention, a process creates a plurality of output images of a field representing one or more wavebands. The process has a first step of providing an optical sensor mounted to a moveable platform, the optical sensor comprising a telescope in functional relationship with a focal plane array, the focal plane array comprising one or more sets of detectors capable of receiving the wavebands and producing focal plane array data of the wavebands, respectively. Next, the field is repeatedly scanned by the focal plane array resulting in scanning data for each detector. Further, a subframe array is generated from the scanning data for each waveband for each scan performed, resulting in a total number of subframe arrays. Also, offset movement data of the focal plane array is generated that corresponds to an approximated line-of-sight for each subframe array. The subframe arrays are integrated into a stabilized waveband array for each waveband using the offset movement data. A fixed pattern for each detector is estimated by integrating the scanning data for each detector and dividing the integrated scanning data by the total number of subframe arrays. The fixed pattern for each detector is suppressed from the stabilized waveband arrays by deducting from each stabilized waveband array element the fixed pattern for each detector for each incidence that the detector contributed to each stabilized waveband array element, thereby generating fixed pattern suppressed stabilized waveband arrays for each waveband. The fixed pattern suppressed stabilized waveband arrays are outputted as the output images.
In a further aspect of the invention, the focal plane array comprises a plurality of sets of detectors and the sets of detectors are intermingled to form a pattern. In a still further aspect of the invention, the pattern is stripes, checkers, zigzags, or random. In an additional aspect of the invention, there are two sets of detectors and the pattern is alternating stripes.
In a further aspect of the invention, the stabilized waveband arrays are inertially stabilized.
A further aspect of the invention comprises the step of normalizing the fixed pattern suppressed stabilized waveband arrays by dividing each fixed pattern suppressed stabilized waveband array element by a respective contribution number, wherein the respective contribution numbers are an amount of incidences that each fixed pattern suppressed stabilized waveband array element was contributed to by the subframe arrays.
In a further aspect of the invention, the repeatedly scanning step is performed 400 times at 4000 Hz.
In a further aspect of the invention, the optical sensor is maintained within predetermined limits of roll, pitch, and yaw. This may be preformed by an attitude control system that is functionally connected to the moveable platform.
In an aspect of the invention, the optical sensor is maintained within a predetermined angular velocity range. This may be preformed by an attitude control system that is functionally connected to the moveable platform.
In an aspect of the invention, the line-of-sight of the optical sensor may be changed relative to the moveable platform.
In an aspect of the invention, the moveable platform is a missile, the telescope, a satellite, a space vehicle, an air vehicle, an aircraft, a ground vehicle, or a watercraft.
In an aspect of the invention, the generating offset movement data step is at least partially performed by an optical sensor line-of-sight measuring device that is functionally connected to the moveable platform.
In an aspect of the invention, the generating offset movement data step is at least partially performed by an inertial reference unit functionally connected to the moveable platform.
In an aspect of the invention, the inertial reference unit comprises one or more gyroscopes.
In an aspect of the invention, the process is not performed using scan mirrors, gimbals, or position pickoffs.
An aspect of the invention involves a process for manufacturing a focal plane array for an optical sensor comprising the steps of providing a plurality of sets of detectors, wherein each set of detectors detects a different waveband and constructing the focal plane array using the plurality of sets of detectors such that the sets of detectors are intermingled in the focal plane array in a pattern.
In an aspect of the invention, the constructing step further comprises the step of arranging the plurality of sets of detectors in alternating stripes. In a still further aspect of the invention, there are two sets of detectors.
In an aspect of the invention, the constructing step further comprises the step of arranging the plurality of sets of detectors in a checkered, zigzag or random pattern.